1. Field of the Invention
The present invention relates to a zoom lens suitable for a still camera, a television camera, a video camera, a photographic camera, a digital camera, etc. More particularly, the present invention relates to a zoom lens using a focusing method that can properly correct aberration over the entire object distance from infinity to the closest photographic distance.
2. Description of the Related Art
As a focusing method adopted in a zoom lens, there is a front focusing method in which a first lens unit located closest to the object side is moved in the optical axis direction. Also, there is an inner focusing or rear focusing method in which a second lens unit or a subsequent lens unit is moved in the optical axis direction. In a focusing method that moves one lens unit, aberration variations are likely to occur during focusing, and it is difficult to obtain high optical performance over the entire focusing range. Particularly, when the object distance is short as required in a macro lens system, large aberration variations occur, and thus it is difficult to maintain high optical performance.
In the current state of the art, there is a zoom lens using a floating method in which a plurality of lens units are moved in the optical axis direction for focusing. The floating method is considered an effective focusing method to suppress aberration variations. In the floating method, the moving amounts of the focus lens units for focusing on an object located at a close distance can be reduced. This reduces the size of the entire lens system, and also reduces aberration variations. In particular, focusing to the close distance, as in macro photographing, can be easily performed while reducing aberration variations.
Japanese Patent Laid-Open No. 61-236516 discloses a zoom lens including, in order from the object side to the image side, a front group having a positive refractive power, a zooming group that moves for zooming, and a relay lens group that does not move for zooming. In this zoom lens, two lens units in the relay lens group are moved for focusing.
A zoom lens disclosed in U.S. Pat. No. 5,701,204 includes, in order from the object side to the image side, a first lens unit having a positive refractive power, a second lens unit having a negative lens unit, and a final lens unit having a negative refractive power. In this zoom lens, the first lens unit and the final lens unit are moved for focusing.
Japanese Patent Laid-Open No. 11-258506 discloses a zoom lens including, in order from the object side to the image side, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, a third lens unit having a positive refractive power, a fourth lens unit having a negative refractive power, and a fifth lens unit having a positive refractive power. The first lens unit and the second lens unit are moved for focusing.
Japanese Patent Laid-Open No. 62-153914 discloses a zoom lens including, in order from the object side to the image side, a front group having a negative refractive power, a zooming group that moves for zooming, and a relay lens group that does not move for zooming. In this zoom lens, two lens units in the relay lens group are moved for focusing.
The floating method is effective in reducing aberration variations in focusing. However, when a plurality of lens units in the zoom lens are simply moved for focusing, it is difficult to reduce aberration variations and to maintain high optical performance.
For example, it is important to properly set the layout of lens units to be moved for zooming, refractive powers of a plurality of lens units to be moved for focusing, and the moving amounts of the lens units. If these settings are improper, it is difficult to obtain high optical performance over the entire focusing range while maintaining a predetermined zoom ratio.